1. Field of the Invention
The present invention relates to an image forming apparatus capable of changing the magnification at which an image of an original is formed on a photosensitive member.
2. Description of the Prior Art
A variable magnification image forming apparatus, such as a variable magnification copying machine, is usually provided with lens means and mirrors for projecting an image of the original on the photosensitive member, which are displaceable to change the magnification at which the image of the original is formed.
Conventionally, plural stoppers are utilized to stop the optical elements selectively at one of several predetermined positions in dependence on the magnification selected. However, it is cumbersome to fix all of the stoppers at respective correct positions. Further, where numerous stop positions are required, or where the stop positions are substantially continuous, a number of, or a great number of stoppers must be used. It is difficult to arrange such numerous stoppers and correctly control the stopping positions.
To obviate those problems, it is good to control the amount of the optical element displacement on the basis of the count of the pulses which are generated as a series of pulses by a pulse generator.
However, there still remains a problem that, even if the driving means for the optical elements are stopped upon the count of the predetermined number of pulses, the optical elements are undesirably stopped at the positions deviated from the correct positions corresponding to the selected magnification, due to the inertia of the optical elements and/or due to non-uniform rotation or the like of the driving means resulting in the deviation between the count of the pulses and the extent of the rotation. The deviation will be integrated with the repeated magnification changes, so that the image is not formed at the selected magnification, or the sharpness of the image will become out of the tolerable range.